From the German patents DE No. 32 31 063 Cl and No. 33 38 608, keys are known which include a carrier which is of electroconductive material or is provided with a conductive coating on which an insulating layer is applied.
The insulating layer is formed with a matrix-like pattern of blank spaces which are closed by a printed insulating material or are kept open by omitting the insulating print. Thus, the blank spaces provide binary information which either permit flow of an electric current to the carrier or block flow.
By inserting the key provided with such coded blank spaces, the information is read in a galvanic manner by means of reading contacts arranged parallel to each other in a single row in the reader.
In order to optically mask the coding for an outside viewer, the blank spaces of the insulating layer after being coded in the described manner are filled with a further conductive layer.
Such a coded locking system has the disadvantage that with simple means--e.g. an ohmeter--the optically masked coded information of the key can easily be measured and can thus be transmitted to another key card so as to copy the key.
The transmission of the code to another key can be obtained by simply changing the codes within the blank spaces of the insulating layer until complete correspondence with the key which is to be copied is achieved.
In case a code element is electrically connected with the carrier but should be insulated, the insulation can be obtained by applying an insulating lacquer with a brush on this code element. If the code element is insulated towards the carrier but should be conductive, it is sufficient to scratch the layer until reaching the carrier and then to provide a connection to the surface of the code element by means of a conductive lacquer e.g. conductive lacquer of silver.
Such a key can thus easily be copied so that its use in a security system is rather limited.